1. Field of the Invention
This invention relates generally to projection systems, and more particularly to a projection system having an illumination source with an offset lens array to eliminate vertical banding in the projected image.
2. Description of the Background Art
Multi-color projection systems typically employ an illumination source to generate a multi-colored illumination beam, and some means (e.g., dichroic plates) to separate the multi-colored illumination beam into a plurality of colored beams. Each of the colored beams is individually modulated with an imager (e.g., liquid crystal display, deformable mirror device, etc.) to infuse the colored beam with an image. The modulated colored beams are passed through some type of analyzer to resolve the image, and then recombined to form a multicolored imaging beam. The multi-colored imaging beam is then projected onto a display screen with a projection lens.
The quality of the images generated by such projection systems depends, in part, on having uniform brightness over the entire projected image. However, vertical bands of varying brightness have been observed in some multi-color projection systems where crossed dichroic plates are used for color separation and/or recombination.
But for the problem of vertical banding, crossed dichroic plates are desirable for use in multi-color systems for reasons of cost and efficiency. What is needed, therefore, is a system and method for eliminating vertical banding in multi-colored projection systems that include crossed dichroic plates.
The present invention overcomes the problems associated with the prior art by providing an offset lens array that reduces banding in optical imaging systems. The invention facilitates the use of optical elements having seams therein (e.g., crossed dichroic plates, color cubes, etc.). The mechanism of and a solution to the problem of banding in projection systems is disclosed.
The offset lens array includes a plurality of lens elements arranged in a plurality of rows, at least two of the rows being horizontally offset with respect to one another. In a particular embodiment, adjacent rows of lens elements are horizontally offset with respect to one another. In a more particular embodiment, each row of lens elements is horizontally offset with respect to every other row in the array. In an even more particular embodiment, starting with a top row of the array, each successive row of lens elements is horizontally offset with respect to the preceding row of lens elements by the same distance. Particularly good results are obtained when the offset distance between successive rows is selected to be equal to one-half the width of one lens element divided by the number of rows in the array.
The offset lens array of the present invention is incorporated in an illumination source of a projection system. The illumination source includes a light source, a parabolic reflector, a first offset lens array, a second offset lens array, and a polarization conversion element. The first lens array has a corresponding arrangement with respect to the second lens array, such that each lens element of the first lens array focuses the image of the light source through an associated element of the second lens array. The polarization conversion element separates the s-state polarization component from the p-state component, and rotates the p-state component to match the emergent, reflected s-state. In a particular embodiment, the offset of the rows in the first and second lens arrays form nonvertical (i.e., not perpendicular to the rows) columns of lens elements. The polarization conversion element is rotated to align the polarization channels in the conversion element with the nonvertical rows of lens elements in the lens array.
The illumination source is incorporated in a multi-color projection system that also includes a color separating element. The color separating element (e.g., crossed dichroic plates) has a seam formed therein, that in prior art systems results in intensity banding. However, because the rows of lens elements of the offset lens arrays of the present invention are arranged asymmetrically with respect to the seam in the color separating element, banding is substantially reduced. The inventors have observed a significant reduction in vertical banding when the offset lens arrays are used in an off-axis, reflective LCD projection system.